Is Optogenetic Activation of Vglut1-Positive Aβ Low-Threshold Mechanoreceptors Sufficient to Induce Tactile Allodynia in Mice after Nerve Injury?

Abstract

Mechanical allodynia is a cardinal feature of pathological pain. Recent work has demonstrated the necessity of A␤-low-threshold mechanoreceptors (A␤-LTMRs) for mechanical allodynia-like behaviors in mice, but it remains unclear whether these neurons are sufficient to produce pain under pathological conditions. We generated a transgenic mouse in which channelrhodopsin-2 (ChR2) is conditionally expressed in vesicular glutamate transporter 1 (Vglut1) sensory neurons (Vglut1-ChR2), which is a heterogeneous population of largesized sensory neurons with features consistent with A␤-LTMRs. In naive male Vglut1-ChR2 mice, transdermal hindpaw photostimulation evoked withdrawal behaviors in an intensity-and frequency-dependent manner, which were abolished by local anesthetic and selective A-fiber blockade. Surprisingly, male Vglut1-ChR2 mice did not show significant differences in light-evoked behaviors or realtime aversion after nerve injury despite marked hypersensitivity to punctate mechanical stimuli. We conclude that optogenetic activation of cutaneous Vglut1-ChR2 neurons alone is not sufficient to produce pain-like behaviors in neuropathic mice. Mechanical allodynia, in which innocuous touch is perceived as pain, is a common feature of pathological pain. To test the contribution of low-threshold mechanoreceptors (LTMRs)to nerve-injury-induced mechanical allodynia, we generated and characterized a new transgenic mouse (Vglut1-ChR2) to optogenetically activate cutaneous vesicular glutamate transporter 1 (Vglut1)-positive LTMRs. Using this mouse, we found that light-evoked behaviors were unchanged by nerve injury, which suggests that activation of Vglut1-positive LTMRs alone is not sufficient to produce pain. The Vglut1-ChR2 mouse will be broadly useful for the study of touch, pain, and itch. in the setting of nerve-injury-induced neuropathy, optogenetic activation of Vglut1-ChR2 neurons did not produce pain-like behaviors such as licking, jumping, or vocalization and also did not produce aversion, suggesting that Vglut1-expressing A␤-LTMRs alone may not be sufficient to produce pain, even under pathological conditions.

Perineural application of resiniferatoxin on uninjured L3 and L4 nerves completely alleviates thermal and mechanical hypersensitivity following L5 nerve injury in rats

“…This provides new evidence that, contrary to the canonical model, neuropathic pain is not only supported by myelinated afferents. Our results are supported by a recent work of Chamessian et al () who reported that ontogenetic stimulation of myelinated nerve fibers alone in transgenic mice was not sufficient to produce pain (Chamessian et al, ). Furthermore, although allodynia is one of the main clinical manifestations of neuropathic pain in human, it has also been described in animal studies (Cheng et al, ; Field, Bramwell, Hughes, & Singh, ).…”

Microglial refinement of A-fibre projections in the postnatal spinal cord dorsal horn is required for normal maturation of dynamic touch

“…In the developing spinal cord, tactile-encoding A-fibres initially project throughout the dorsoventral extent of the dorsal horn and refine over the first few postnatal weeks to terminate in deeper laminae, segregated from the more superficial terminals of noxious-encoding C-fibres 3 . We confirmed this using a transgenic reporter mouse in which tdTomato is expressed in vesicular glutamate transporter 1 (Vglut1) expressing neurons (VGLUT1-tdT), a population of large myelinated sensory neurons with features consistent with Aβ-low threshold mechanoreceptors (LTMRs) 9 . The central axon terminals of these neurons extend into the superficial laminae in the early postnatal period (Fig.…”

Excitatory neurons are more disinhibited than inhibitory neurons by chloride dysregulation in the spinal dorsal horn

“…Natural tactile stimuli will co-activate both types of spinal neurons, but may do so in different proportions depending on stimulus characteristics. Recent efforts using optogenetics to identify which type of low-threshold mechanoreceptors mediate allodynia after nerve injury are interesting in this regard: Dhandapani et al (2018) showed that activating TrkB-ChR2-positive neurons – which include D-hair (Aδ) and RA afferents – provokes allodynia whereas Chamessian et al (2019) showed that activating Vglut1-ChR2-positive neurons – which include RA and SA afferents – does not. The discrepancy suggests that activation of RA but not SA afferents produces allodynia, which implies that spinal circuits implement a NIMPLY logic gate.…”